In a coal fired boiler apparatus for a thermal power generation burning a pulverized coal as a fuel, a vertical crusher is used in a fuel supply apparatus.
FIG. 21 is a view of an outline structure of a conventional vertical crusher, FIG. 22 is a view of a partial outline structure of a classifier provided in the vertical crusher, and FIG. 23 is a cross sectional view on a line X-X in FIG. 22. The vertical crusher is mainly constituted by a crushing portion 5 crushing a coal 50 corresponding to a raw material of a pulverized coal on the basis of an engagement between a crushing table 2 and a crushing ball 3 (or a crushing roller), and a classifier 6 installed in an upper portion of the crushing portion 5 and classifying the pulverized coal to an optional grain size.
Next, a description will be given of an operation of the vertical crusher. The coal 50 corresponding to a crushed material supplied from a coal supply tube 1 comes down to a center portion of the rotating crushing table 2 as shown by an arrow, thereafter moves to an outer peripheral portion while drawing a spiral locus on the crushing table 2 on the basis of a centrifugal force generated together with the rotation of the crushing table 2, and is engaged between the crushing table 2 and the crushing ball 3 so as to be crushed.
The crushed particles are blown up to an upper side while being dried by a hot wind introduced from a throat 4 provided in the periphery of the crushing table 2. The particles having a large grain size in the blown-up particles come down due to a gravity in the middle of being carried to the classifier 6, and are returned to the crushing portion 5 (a primary classification).
The group of particles reaching the classifier 6 are classified into the fine particles having a grain size equal to or smaller than a predetermined grain size, and the coarse particles having a grain size larger than the predetermined grain size (a secondary classification), and the coarse particles come down to the crushing portion 5 so as to be crushed again. On the other hand, the fine particles getting out of the classifier 6 are fed to a coal fired boiler apparatus (not shown) from a discharge pipe 7.
The classifier 6 is formed as a two-stage structure comprising a fixed type classifying mechanism 10 and a rotary type classifying mechanism 20. The fixed type classifying mechanism 10 has a fixed fin 12 and a recovery cone 11. The fixed fin 12 is suspended downward from a ceiling wall 40 as shown in FIGS. 21 and 22, and a lot of fixed fins 12 are fixed at an optional angle with respect to a direction of a center axis of the classifier 6 as shown in FIG. 23. The recovery cone 11 is provided in a bowl shape in a lower side of the fixed fin 12.
The rotary type classifying mechanism 20 has a rotating shaft 22, a rotating fin 21 supported to the rotating shaft 22, and a motor 24 rotationally driving the rotating shaft 22. The rotating fin 21 is structured such that a longitudinal direction of a plate extends approximately in parallel to a direction of a center axis (a direction of the rotating axis) of the classifier 6, and a lot of rotating fins 21 are arranged at an optional angle with respect to the direction of the center axis of the classifier 6 as shown in FIG. 23, and rotate in a direction of an arrow 23.
As shown in FIG. 22, a solid and gas two-phase flow 52 constituted by a mixture of solid particles and gas blown up from a downward side so as to be introduced to the classifier 6 is first rectified at a time of passing through the fixed fins 12 and a weak swing motion is previously applied at the same time (refer to FIG. 23). Further, a strong swing motion is applied at a time of reaching the rotating fins 21 rotating at a predetermined rotating speed around the rotating shaft 22, and a force flipping the particles to an outer side of the rotating fins 21 is applied to the particles in the solid and gas two-phase flow 52 on the basis of a centrifugal force. Since the great centrifugal force is applied to the coarse particles 53 having a great mass, the coarse particles 53 are separated from the air flow passing through the rotating fin 21. Further, the coarse particles come down from a portion between the rotating fins 21 and the fixed fins 12 as shown in FIG. 22, and finally slide on an inner wall of the recovery cone 11 so as to come down to the crushing portion 5.
On the other hand, the fine particles 54 pass through the portion between the rotating fins 21 rotating together with the air flow due to its small centrifugal force, and are discharged as product fine powders to an outer portion of the vertical crusher. A grain size distribution of the product fine powders can be adjusted by a rotating speed of the rotary type classifying mechanism 20. In this case, reference numeral 41 denotes a housing of the crushing portion 5.
In the product pulverized coal supplied to the coal fired boiler apparatus, a pulverized coal in which a grain size distribution is sharp and the coarse particles are hardly mixed is required, for reducing air pollutants such as nitrogen oxide (NOx) or the like and a cinder unburned combustible. Specifically, it aims at making a mixed rate of the coarse particles of 100 mesh over equal to or less than 1 weight % in the case that a mass rate of the fine particles of 200 mesh pass (a grain diameter equal to or smaller than 75 μm) is 70 to 80 weight %.
The following patent document 1 describes a classifier which can reduce the mixing rate of the coarse particles of 100 mesh over in comparison with the conventional classifier. FIG. 24 is a view of a partial outline structure of the classifier.
The classifier is provided with a cylindrical downward flow forming member 13 suspended from an upper surface plate 40 in an outer peripheral side of the rotating fins 21. The solid and gas two-phase flow 52 coming up from the crushing portion ascends to the below of the upper surface plate 40 on the basis of an inertia force. Further, the flow comes to a downward flow moving downward on the basis of the gravity after passing through a gap of the fixed fins 12 and coming into collision with the downward flow forming member 13. When the flow changes to the flow toward the rotating fins 21 side near a lower end portion of the downward flow forming member 13, the coarse particles 53 having the great gravity and the great downward inertia force are separated from the flow, and come down to the lower portion along the inner wall of the recovery cone 11. Accordingly, the group of particles hardly including the coarse particles 53 reach the rotating fins 21, and it is possible to reduced the mixing rate of the coarse particles in the product fine particles.
The following patent document 2 describes defining proper length and position of the downward flow forming member 13.
Patent Document 1: JP-A-10-109045
Patent Document 1: JP-A-2000-51723